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Black Walnut Trees: The Roots of Evil

Could this tree be toxic to your garden?

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The black walnut tree is one of our most valuable and beautiful native trees. The easily worked, close-grained wood has long been prized by furniture- and cabinetmakers for its attractive color and exceptional durability. Its logs are in such demand for veneer that "walnut rustlers" have made off with trees in the dead of night and even used helicopters in their operations.

The early settlers discovered black walnuts growing in mixed forests from Canada to northern Florida and west to the Great Plains. They found that its rich-brown heartwood was exceptionally resistant to decay and put it to use as fence posts, poles, shingles, and sills. When surrounded by other trees in the forest, black walnut grows straight and tall with few, if any, lower branches. When planted in the open, the tree will branch out closer to the ground, developing a spreading shape that makes it easier to harvest its sweet, round, two- to three-inch nuts. The settlers snacked on the nutritious nuts out of hand, added them to soups and stews, and ground them into meal for baking; the hard shells provided a perfect package for storing the nuts over winter.

Unfortunately, the black walnut does have a dark side. Its roots, which may extend 50 feet or more from the trunk, exude a natural herbicide known as juglone that prevents many plants from growing within their reach. Tomatoes, potatoes, apples, pears, berries, and some landscape plants such as rhododendrons, azaleas, and lilacs may be killed or stunted if grown in close proximity to black walnut roots.

In spite of this, black walnuts make great shade trees for larger properties. They commonly grow to 50 feet or taller and about as wide, and specimens of more than 100 feet have been recorded. Black walnut's large, fernlike foliage provides light, airy shade for those grasses and ground covers not affected by juglone. In autumn, the leaves turn bright yellow, contrasting nicely with the tree's rugged, dark bark.

Black walnuts require a deep, fertile soil with a near-neutral or slightly acidic pH. They are pretty much disease-free and are threatened by few pests, with the exception of perhaps an occasional helicopter.

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Comments

I have a few acres of black

By Corinna

I have a few acres of black walnut trees. Unfortunately they were planted about 10 feet apart so they have grown tall and thin. They are about 50 feet tall with trunk diameters of 6-12inches. I'd like to thin some areas in hopes that the trees will branch out and spread at the crown. Is it too late for them or will opening the area encourage them to branch out?

Thanks!

Hi, Corinna: This is a good

By Almanac Staff

Hi, Corinna: This is a good question, with answers so complicated that we dare not go there. Some of this depends on the purpose of the trees (harvest anything? shade? beauty?). In any event, walnut trees with this extremely tight spacing are best thinned when their diameters are 3 to 4 inches, not 6 and up. Your local climate and soil conditions might come into play, too, not to mention the extensive root systems. We recommend consulting with a professional arborist. Good luck!

Poison ivy vines are growing

By Mary Mcclintock

Poison ivy vines are growing exclusively on all my walnut trees in the unkept, mature woods of the older home I just bought in northern MD. Wild grape, oak, and rosa flora vines are grwing on all the other trees but never on the walnuts. Why? I love and hate it. How agressive can I get with herbicides and not hurt the trees?

Hi, Mary: This seems to be a

By Almanac Staff

Hi, Mary: This seems to be a question not only of arboriculture, but also of semantics. Why poison ivy is not growing on your other trees, we don't know. Why the grape, etc., are not growing on your black walnuts is because b-w's produce an herbicidal substance called juglone (see our intro above) that is toxic to many plants. It is not toxic to poison ivy, which is why p-i will grow on black walnuts. In and of itself, a good (p-i specific, if possible), organic herbicide is not going to kill the trees. In and of itself, it is also not going to kill the p-i vines. The thing to do is first to thoroughly cover every inch, nook, and cranny of yourself with protection from p-i's urushiol oil. Let us repeat that: Cover yourself totally. Then carefully remove (cut out) sections (a foot long is good) of the vines, which will kill what is above (but the oil will still be potent for a year or two). Then, focus on spraying what is below the cut. Carefully discard the cut sections in a way that no one can touch them or what they have touched. By the way, though: Poison ivy leaves are edible by quite a range of wildlife. It's a complicated situation, and no doubt this reply has just scratched the surface.

I just recently found out I

By imstunned

I just recently found out I have a black walnut tree in my next door neighbors home right on my fence line. It drops these green balls onto my roof and scares the crap out of me lolz...I grabbed one today threw it on my cement patio it broke the green ball and I seen it was a walnut shell on the inside. Now I live in northern illinois and my neighbors never planted the tree. How the heck is it surviving?

Normally, you need a special

By Almanac Staff

Normally, you need a special tool to crack a black walnut shell (that is stronger than a common nut cracker) but I guess you found a creative solution! :-) Inside is a sweet nut that you can eat or make into pies.  The tree may have seeded itself; it's hard for us to know. Just keep in mind that the area near the tree can be toxic to other plants and vegetables, so something to keep in mind when you consider whether you want to keep it.

I have a whole yard full of

By timtim

I have a whole yard full of black walnut trees and a garden right next to one and my plants don't get toxic as a matter of fact I had more tomatoes than I knew what to do with.

We have a Black walnut tree

By Debb85

We have a Black walnut tree in our front yard in Lincolnton, N.C. My question is we have been living here 5 years, and this is the first year it is not bearing nuts. Do they have a non-bearing year like the Pecan tree. Thank you

Some black walnut cultivars

By Almanac Staff

Some black walnut cultivars are indeed prone to alternate bearing or just bearing more heavily every other year.
If your leaves seem unhealthy, here is also a fungus that can effect nut production but can be controlled by fungicide applications. You could contact your cooperative extension for further diagnosis.

My question is when you first

By Terry R Lott

My question is when you first plant your black walnut tree and its only a foot tall,will the deer bite the tops off of them and kill the tree ?

I have or I think I have

By Tim Zydiak

I have or I think I have black wallnut trees on my prop. about 35 feet apart. For 15 years I,ve lost many pine trees that I've planted around these black wallnut trees. Is there a pine tree that resists the JUGLONE that was talked about?

I take exception to your

By Wm. Burnett

I take exception to your comment regarding Black Walnut pest.
You mention few are out there, we have 17 walnut trees in our back yard.
We live in S.W. Ohio 2.7 miles from a large Veneering company. They are suspect of importing diseased walnut trees from Colorado. We have a major infestation of Walnut Twig Beetles (size of a sesame seed) which carry a fungus which causes 1000 Canker Disease.
Our situation is so bad the state of Ohio Agriculture Dept. has conducted training classes in our yard. We've had Professors from Ohio State University and University of Tennessee investigating and taking samples. Plus the Ohio Division of Forestry Department had 25 of their personal here for observation.
We currently have one of in excess 110 traps around the county. These traps are not to catch the beetles for disposal rather they are for verification of the infestation in other areas.
It's now up to us to take down these dead or dying trees. Then we must figure out a means of disposal without removing any portion including saw dust from the county of which we live Butler County Ohio.
I decided to pass this information on in the event your Walnut trees decided to start dropping leaves early and often all summer long including.

Thanks for your comment. This

By Almanac Staff

Thanks for your comment. This column was written in late 2005 or early 2006 and we appreciate your update.

The walnut twig beetle is a pest that seems to be increasing among black walnut trees (primarily affects black walnut Juglans nigra). Tinier than a grain of rice these beetles carry the fungus Geosmitha morbida that eventually (within a couple or few years) infects the tree and starves it to death. See: http://www.aces.edu/ucf/BlackwalnutThousandCankerDisease.php
Disposal of the remains can be a complicated and costly procedure.

Do you need to fertilize the

By Melissa Giovagniolli

Do you need to fertilize the soil and also what pesticide do you need to treat the black walnut trees? Thanks

It would probably depend on

By Almanac Staff

It would probably depend on whether the black walnuts grow naturally in your area. If they do, we'd say you needn't bother fertilizing, unless you are setting up a stand of trees to harvest the nuts or wood, or if the tree begins to look unhealthy. When fertilizing, we'd recommend that you test your soil for nutrients and then contact your county's Cooperative Extension or an arborist to see what fertilizer is needed for your local soil, for black walnuts, and for the age of the tree. Soil pH is best around 6.2 to 7.2.

 

The same would apply to pesticides. If the tree is native to your area, and you are just interested in the tree as a landscape feature, then it would not need pesticides unless a specific problem arises. If, however, you are planning to harvest, then monitoring for specific pests, such as the walnut husk fly, would be called for, with the appropriate pesticide that is safe for edibles if harvesting the nuts rather than the wood (check with an arborist or your local garden center for what's appropriate in your area).

 

For more information, you might be interested in the following:

 

http://www.walnutcouncil.org/resources/growing-hardwoods/fertilization.pdf

 

http://extension.usu.edu/files/publications/publication/HG_HomeOrchard_2...

 

http://web.extension.illinois.edu/forestry/publications/pdf/black_walnut...

 

http://www.walnutcouncil.org/resources/growing-hardwoods/insects-and-dis...

I live in a neighborhood that

By Ryan Adams

I live in a neighborhood that has many mature trees and I have mulitple black walnuts on my property. They are mature but I do not harvest them. The past few years I have been losing many large and small branch's some in storms but most are brought down by the slightest wind gusts. Is this normal? Is there something I can do to prevent it?

This can happen when there is

By Almanac Staff

This can happen when there is too much weight on the ends of the branches. Just prune to thin your trees. Be careful not to overprune or strip the tree on one side or this can cause the tree to get weakened and the branches to bend and break, too.

I bought a large farm 2 years

By Tom Cherry

I bought a large farm 2 years ago (2011) and dicovered about 8 fully grown walnut trees on the edge of the fields in the woods. I am going to try a harvest this year for pesonal consumption. It is august now and was wondering what a good time to and how do I know the best time for harvest. I am in Maryland.

Black walnuts are harvested

By Almanac Staff

Black walnuts are harvested in the fall (mid to late September) once the hull changes from a solid green to a yellowish color and is soft enough that you can dent it with your thumb. The walnuts should ripen on the tree, so you can shake the tree or just wait until the nuts fall. Here is a good reference page on how to harvest and prepare the walnuts:
http://www.extension.umn.edu/yardandgarden/ygbriefs/h404blkwal.html

My neighbor has a huge Black

By wellheck

My neighbor has a huge Black Walnut on the border between our properties. It is exactly where it does us the least good, because it is overshadowing an area that would be perfect for a garden. I call it the "Black Widow Tree".

We have a very old very large

By loosey

We have a very old very large black walnut. It is a lovely tree but drops yellow spotted leaves all summer. Keeps lawn covered and messy.

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